Scanning x-ray imaging systems are commonly used for the inspection of packages, luggage, cargo containers and vehicles. Commercially available scanning x-ray imaging systems can be broadly categorized into two types: flying-spot systems and line-scan systems. Flying-spot systems employ a “pencil” beam of radiation that is rapidly scanned over the object of interest. Such systems are capable of measuring either or both of transmitted and backscattered radiation. The pencil beam is formed by collimation (in two orthogonal dimensions), because practical methods for focusing x-rays of the required energy range are not available. Since the formation of the pencil beam excludes all but a tiny fraction (typically much less than one percent) of the available source flux, a flying-spot system requires a high power X-ray source to produce an image having acceptable resolution and signal-to-noise ratio.
Line scan systems utilize a “fan” beam of radiation to illuminate the object under inspection and a segmented detector to measure radiation transmitted through the object. Commercial line-scan systems, while advantageously using a much higher portion of the available source flux, are generally incapable of producing images from backscattered radiation, and hence their use is limited to applications where it is practical to position the radiation source and the detector on opposite sides of the inspected object and where detection of targets composed of light elements is not paramount.
The prior art contains several references that disclose x-ray imaging systems which attempt to take advantage of the relatively efficient source usage of line-scan systems while maintaining backscatter imaging capabilities. Representative examples of such references include U.S. Pat. No. 6,453,007 to Adams et al., which teaches a specially-shaped chopper wheel for rapidly alternating the illumination beam between fan and pencil shapes, and U.S. Pat. No. 6,269,142 to Smith, which teaches a line-scan imaging system adapted with a rotating beam stop that periodically interrupts the fan beam. In another approach, described in U.S. Patent Application Publication No. 2002/0031202 to Callerame et al., the inspected object is illuminated with a scanned set of pencil beams or a fan beam divided into sections, wherein each pencil beam or fan beam section is encoded by modulation with a unique characteristic frequency. In this manner, each simultaneously illuminated pixel-sized segment of an inspected area of the object may be associated with a different characteristic frequency in the detector signal, such that the detector signal may be demodulated (e.g., by using a filter bank) to recover spatial information and construct an image of the inspected region.
The method of coded aperture imaging is known in the art, and is has been used for gamma-ray and x-ray astronomy, radioactive materials management, nuclear medicine, and other applications involving non-focusable radiation. In a typical configuration, one or more radiation sources project a pattern through a coded mask onto a pixilated (segmented or otherwise position sensitive) radiation detector. An image of the source is then reconstructed from the projected pattern through a decoding algorithm. The coded aperture imaging method has the potential for improved sensitivity (relative to other known imaging methods such as “pinhole” imaging) by allowing radiation to arrive at the detector through a coded mask of large area and high openness (typically up to about 50% of the mask area). The mathematical techniques for encoding and decoding are well established, and are described, for example, in U.S. Pat. No. 4,209,780 to Fenimore et al., U.S. Pat. No. 5,606,165 to Chiou et al., and U.S. Pat. No. 6,737,652 to Lanza et al., as well as in Fenimore et al., “Coded Aperture Imaging With Uniformly Redundant Arrays,” Applied Optics, 17(3): 337-347 (1978). A variety of encoding methods are available, including (without limitation) the following: uniformly redundant array (URA), modified uniformly redundant array (MURA), product array, m-sequence, pn-sequence and Hadamard difference set.
Straightforward application of traditional coded aperture imaging to neutron activated gamma-ray emission and to x-ray backscatter has been respectively proposed in U.S. Pat. No. 5,930,314 to Lanza and U.S. Patent Application Publication No. 2004/0218714 to Faust. Backscatter detection by the traditional coded aperture imaging method allows the source to flood the entire inspection area simultaneously for efficient use of the available source flux. For this method, pixilated detectors are required, and for high system performance these detectors must have large areas and fine segmentation.
Several variations on the traditional coded aperture imaging method have been described in the prior art. U.S. Pat. No. 5,940,468 to Huang et al. teaches the use of a fan beam to illuminate the inspected object, with plural large area coded masks interposed in the backscattered radiation path between the object and corresponding large-area detectors. This approach makes efficient use of the available source flux, but requires its large-area detectors to be finely segmented along one axis. U.S. Pat. No. 6,950,495 to Nelson et al. teaches a beam encoding scheme based on a wide-area radiation source in which the image of the inspected object is decoded from a series of backscatter responses to different source patterns. This scheme makes poor use of the available source flux because it requires the modulated source radiation to pass through a pinhole aperture en route to the inspected object. Furthermore, it is believed that the wide-area source required to implement the Nelson et al. scheme will be overly bulky, heavy, complex and expensive. Finally, U.S. Pat. No. 7,136,453 to Jupp et al. teaches a backscatter imaging system using a stationary coded mask with a source spot that moves over a planar area in a raster-scan manner. While this system generally makes efficient use of the available source flux and does not require segmented detectors, it is believed that a system of this general description would be prone to image distortion due to a number of factors, including, inter alia, backscatter from structures at the periphery of the scanned volume, variation of the path length from the source spot to any particular portion of the mask and any particular portion of the inspected object as the source spot is scanned, vignetting of the coded mask with variation of the source ray's angle of incidence, and parallax in the near field. Furthermore, the scanning x-ray source required by the Jupp et al. scheme would be difficult and expensive to implement.
Against the foregoing background, there remains a need in the art for an imaging apparatus that makes efficient use of available source flux, does not require segmented detectors, avoids or reduces the image distortion problems associated with prior art approaches, and is not prohibitively difficult or expensive to manufacture.